Polymeric materials, particularly thermoplastics such as polycarbonate, are promising alternatives to glass for use as structural material in a variety of applications, including automotive, transportation and architectural glazing applications, where increased design freedom, weight savings, and improved safety features are in high demand. Plain polycarbonate substrates, however, are limited by their lack of abrasion, chemical, UV and weather resistance, and therefore need to be protected with optically transparent coatings that alleviate above limitations in the aforementioned applications.
Thermal curable silicone hardcoats have been widely used to protect thermoplastic substrates. Typically, the coated substrates are made by initially priming the substrate with an acrylate based primer coat, followed by the application of a silicone hardcoat composition to the primed substrate. Primerless hardcoat compositions have also been developed which allow the coating compositions be applied directly onto unprimed thermoplastic substrates. See for example U.S. Pat. No. 5,349,002 to Patel et al.
While silicone hardcoats are useful in improving the abrasion resistance and weatherbility of thermoplastics, they are typically rigid due to the relatively high cross-linked nature of the cured silicone coating composition. Accordingly, bending of the thermoplastic sheets coated with silicone hardcoat beyond certain extent often leads to cracks/micro-cracks in the hardcoat, which makes these silicone hardcoat less desirable for certain applications.
Efforts have been made to improve the flexibility of silicone hardcoats either by lowering their cross-linking density or by adding an organic polymeric additive to the hardcoat compositions.
Addition of a suitable additive into the silicone hardcoat formulation seemed to be an easy option. But the identification of such an additive is not straightforward, since such additive should satisfy a number of different criteria, namely, (1) being polymeric and soluble in solvents used in the silicone hardcoat formulations, (2) having the ability to become part of the cross-linked network of the silicone hardcoats, (3) not affecting the cross-linking reaction between the components of typical silicone hardcoat compositions, and most importantly, (4) not affecting typical silicone hardcoat properties.
Heretofore, many additives disclosed in the art are less than satisfactory since the incorporation of these additives often leads to inferior abrasion resistance performance of the silicone hardcoats.
Accordingly, there is a need for a new additive that is effective to improve the flexibility of the conventional silicone hardcoats while at the same time, does not adversely affect the abrasion resistance, weatherability and other physical properties of conventional silicone hardcoats. The present invention provides an answer to that need.